Plasma Shell Research Articles

A simple model for plasma temperature in imploded hollow plasma liners

Existing theory for the Rayleigh-Taylor instability in imploding hollow plasma liners has assumed a constant electrical resistivity during most of the implosion. While this is qualitatively justified by the competition between joule heating and field-diffusion-driven expansion of the plasma shell; one, nevertheless, expects the temperature and, therefore, electrical conductivity to rise during the implosion. A simple model for plasma temperature as a function of time, based on the neglect of radiative losses and using approximate fits to equation-of-state information, is presented here. The results are used to compute the minimum allowed wavelength, a parameter used to assess instability effects, and agreement with magnetohydrodynamic calculations to well within a factor of 2 is obtained.

Hydrodynamic instabilities in an imploding cylindrical plasma shell

Hydrodynamic instabilities of a cylindrical plasma shell which is imploded by the pressure of an external massless fluid are considered. The plasma shell is assumed to be a compressible, isentropic fluid which is describable as an ideal gas ($p=a{\ensuremath{\rho}}^{\ensuremath{\gamma}},\ensuremath{\gamma}\ensuremath{\ne}1$). The unperturbed plasma shell is assumed to undergo a self-similar motion. This assumption together with mass conservation determines the time-dependent pressure profile of the plasma shell. Linear stability analyses for this prescribed self-similar motion are carried out analytically. Stability criteria are obtained in terms of the ratio of specific heats $\ensuremath{\gamma}$ and an azimuthal mode number of perturbations. It is shown that the imploding plasma shell is unstable to compressible perturbations when $\ensuremath{\gamma}<2$. In the case of incompressible perturbations, similar calculations show that the Rayleigh-Taylor instability occurs at the outer surface and, in addition, the unstable mode develops at the inner surface. A plausible mechanism for the development of the unstable mode at the inner surface is given.

Dynamics of a plasma shell with an outside current

Dynamics of a plasma shell with an outside current

Diffusion of magnetic field into an expanding plasma shell

A simple theory is developed to describe the density profile of an electromagnetically imploded hollow plasma liner. The model gives a good analytic means of determing density gradient scale length and sheath thickness for use in linear and nonlinear analysis of the Rayleigh–Taylor instability.

Laboratory experiments and space phenomena

Two types of convection were observed in the laboratory model of the magnetosphere: viscous convection and convection due to field lines common to both the magnetosphere and artificial solar wind. With a southward field component in the solar wind, convection from the Sun is observed in the polar cap, while with a large northward component, convection is directed toward the Sun. Merging of the field lines occurs in the cleft. With the southward component, a visor appears in front of the magnetosphere boundary. The decay of the visor into small magnetic structure is observed. The formation of an induced magnetosphere with a magnetic tail is shown in the experiments of the simulated conditions near non-magnetic bodies with a plasma shell (Venus, comets). A combined induced-intrinsic magnetosphere also was investigated.

Ignition model for spark discharges and the early phase of flame front growth

A thermal ignition model is derived, taking into account the nonstationary character of the ignition process. It is especially suited for applications such as in spark ignition, as shown by detailed experimental evidence of its main features and by test bench tests. Minimum ignition radii and ignition energies are calculated from basic flame data as well as the early stages of flame expansion. Using reasonable functions for the parameters involved in the theory, it is possible to account for the influences of equivalence ratio, flow or turbulence, preheating, mixture preparation etc. Lean ignition limits for glow discharge and breakdown mode are checked, and an excellent agreement between measured and calculated values is found. Thus it is concluded that ignition energy is used only from the outer shell of the spark plasma. Criteria for the design of optimized ignition systems are derived with direct bearing on practical ignition systems. The results throw new light on the difficulties of measuring minimum ignition energies and on the proper use of values from the literature.

Laboratory simulation of the induced magnetospheres of comets and Venus

The comparison of data obtained in laboratory experiments on the solar wind interaction with a body endowed with a plasma shell, the observations of comet type I tails and the direct measurements near Venus show that an induced magnetosphere is formed with an extended magnetic tail. This magnetosphere appears due to currents associated with unipolar induction. The distribution of electrodynamical forces associated with the formation of the induced magnetosphere makes it possible to explain the acceleration of matter towards the tail as in the motion across the tail observed in comets and Venus. The analysis of the condensation motion in Halley's comet yields an estimate of tail magnetic field of 30 to 50γ. A three-dimensional model of the induced magnetospheres of Venus and comets is developed.

Production of a cylindrical plasma shell

We describe an apparatus which forms a hollow cylindrical plasma shell of 3×2 cm2 cross section 40 cm in diameter by injecting a neutral gas shell of argon via supersonic injection nozzles and ionizing this shell with a high-voltage electrical discharge.

Two-dimensional magnetic flux shell model for magnetohydrodynamic simulations

A magnetic flux shell model is described which is suitable for numerical simulations of cylindrically symmetric plasmas. The magnetohydrodynamic equations in magnetic field line coordinates are derived including the effects of finite electrical conductivity and anisotropic heat conductivity. Subsequently, the equations of motion of coaxial plasma shells of constant density, temperature, axial velocity, and axial magnetic field are derived including (a) full radial dynamics and (b) radial pressure balance. Sample simulations of a laser heated solenoid plasma and a ϑ-pinch reactor are described.

A cylindrically stratified model of the plasma wake of a dipole antenna entering a planetary atmosphere

The radiation characteristics of a dipole antenna are calculated in a high‐velocity wind of ionized gases representing the wake region of an antenna located on the aft portion of a probe entering a planetary atmosphere. The antenna and the support structure are assumed to give rise to a cylindrically stratified plasma flow field, which is allowed to possess a radial distribution in the electron concentration and collision frequency. Each cylindrical layer of the plasma shell moves at a uniform axial velocity with respect to the antenna and is biased magnetically along the axis of the cylinder. The antenna is represented by a circularly polarized turnstile antenna mounted off‐axis a quarter wavelength above a ground plane and is oriented transverse to the axis of the cylinder. The antenna operates at frequencies near the cutoff frequency of the plasma in the wake. Integral expressions for the cylindrical components of the electric and magnetic field intensities are derived and are evaluated in the rest frame of the antenna using the techniques of asymptotic expansions. The resulting far‐field gain patterns of the antenna are given and are compared with the free‐space patterns.

Electromagnetic-implosion generation of pulsed high-energy-density plasma

The generation of pulsed high-energy-density plasmas by electromagnetic implosion of cylindrical foils (i.e., imploding liners or hollow Z pinches) has been investigated experimentally and theoretically at the Air Force Weapons Laboratory. The experimental studies involve discharging a 1.3-μsec 1.1-MJ capacitor bank through 7-cm-radius 2-cm-tall 3–30-mg cylindrical foil liners. Typical discharge parameters are 7–12-MA peak current and 1–1.5-μsec current rise time. Current and voltage waveforms indicate strong coupling of the load to the capacitor bank, and analysis of the waveforms indicates good implosion of the current sheath. Optical- and magnetic-probe measurements are consistent with 1–2-cm thickness of the imploding plasma shell and with final implosion velocities ∼15–20 cm/sec. Radiation-diagnostic measurements indicate ultrasoft x-ray yields ∼50–100 kJ with the FWHM of the photon pulse ∼80–100 nsec. The radiation data is consistent with a quasiblackbody spectrum (T∼30–50 eV) comprising most of the energy, with additional higher temperature and optically thin spectral components. Al11+ and Al12+ line and recombination radiation is frequently observed. Comparison of electrical, magnetic, and radiation data with one-dimensional MHD and two-dimensional MHD calculations is presented. The prospects for improving the performance with the present energy source and scaling to larger energy sources are briefly discussed.

Possibility of rapidly cooling a multiply charged laser plasma and using it as an active medium for generating short-wavelength radiation

In order to increase the rate of cooling of a laser-produced plasma, formed by the breakdown of a gas in a focused light beam, it is proposed to surround it with a plasma shell of substantially lower temperature, produced by means of an additional picosecond pulse. Electron heat conduction in the colder outer region causes the temperature at the center of the plasma to drop. The recombination flux in the central region can thereby be increased considerably at the expense of a relatively small reduction in the plasma density. Estimates show that under such conditions one can expect an appreciable inversion of short-wavelength transitions and lasing in the far vacuum ultraviolet.

Two-dimensional simulation of the hydromagnetic Rayleigh-Taylor instability in an imploding foil plasma

Two-dimensional (r-z) magnetohydrodynamic simulations of the electromagnetic implosion of metallic foil plasmas show, for certain initial configurations, a tendency to develop large-amplitude perturbations characteristic of the hydromagnetic Rayleigh-Taylor instability. These perturbations develop at the plasma magnetic field interface for plasma configurations where the density gradient scale length, the characteristic dimension for the instability, is short. The effects on the plasma dynamics of the implosion will be discussed for several initial foil configurations. In general, the growth rates and linear mode structure are found to be influenced by the plasma shell thickness and density gradient scale length, in agreement with theory. The most destructive modes are found to be those with wavelengths of the order of the plasma shell thickness.

Forced Radial Oscillations of a Fully Ionized Cylindrical Plasma Surrounded by a Partially Ionized Plasma Shell

Forced radial magnetoacoustic oscillations of a fully ionized cylindrical plasma, surrounded by a partially ionized shell are excited by means of an external source. The radial plasma profiles of the macroscopic field quantities in the fully and partially ionized regions are obtained as functions of the degree of ionization, the temperature and the position of the boundary between the two regions. The absorbed power is calculated. The nonresonant as well as the resonant cases are treated. It is shown that the main part of the power is absorbed in the outer shell unles a suitable resonance is used to enhance the macroscopic field quantities in the fully ionized plasma part and the dimensions of the outer shell are kept small.

Increased nuclear energy yields from the fast implosion of cold shells driven by nonlinear laser plasma interactions

The nonlinear interaction force between intense laser fields and cold plasma shells efficiently transforms radiant energy into mechanical energy of implosion. This transfer of energy has been considered before in numerical experiments and it is treated here analytically in a didactic example starting with an inhomogeneous Rayleigh density profile. Up to 50% of the laser energy can be transformed into the energy of compression if a single untailored pulse of 2.5×1O16 W/cm2 intensity and of only a few picosecond duration is used for spherical illumination of a shell. If the pulse is short enough to reduce collisiona) thermalization, then the collapse and compression of the plasma can remain at the threshold of Fermi degeneracy and still be adiabatic. This results in nuclear reaction gains G, based on the deposited energy, E0, and without a-particle reheating, of G = 400 for E0 = 2.25 kJ (D–T reaction), 900 kJ (D–D), 13 MJ (H–B). About 1000 times less laser energy is necessary than in the case of gas dynamic ablation resulting in the same nuclear reaction yields.

Stationary states for electron oscillators in a rare-gas afterglow plasma

An explanation is advanced for the presence of populations of low energy oscillating paired electrons in rare-gas afterglow plasmas arranged in a system of coaxial shells, and enclosing a core plasma of free electrons. The common axis of the system is that of the long glass tube containing the plasma. The explanation is based on Fermi's analysis of the collisional interaction of very low energy electrons with gas atoms or molecules. A model afterglow is evolved based on the coaxial shell hypothesis, and is used to interpret two sets of afterglow phenomena, namely the plasma shell resonances obtained when energy is radiated by electrons making transitions from inner to outer shells, and the multiple values for accelerated decay times observed for afterglows of this type.

Distinguishing Between Thermalizing and Electrodynamic Coupling For Laser-compressed Thermonuclear Reactions

In the gas-dynamic laser compression of plasmas for exothermal nuclear reactions, certain laser intensities must not be exceeded in order to reach sufficient thermalizing coupling. A criterion for this limitation is developed here, which shows that the published cases of Nuckolls (1974) and Brueckner (1974) are close to this threshold and may not need serious changes due to the coupling process. As weII as the long pulse scheme of Afanasyev et al. (1975), an alternative method exists in which very short and very high intensity laser pulses are applied, thereby avoiding thermalization but generating fast imploding cold and thick plasma shells due to nonlinear ponderomotive forces of optical explosion. A necessary and sufficient condition for this electrodynamic coupling is derived, and the general nonlinear force equation is integrated to derive ion energies and their exact linear increase with ion charge. Experiments indicating the predominance of the nonlinear force are then analysed. The derived criteria are used to explain examples of nonlinear force compression giving 1000 times greater efficiencies for nuclear reactions than the gas-dynamic case.

Line radiation in the visible and in the ultraviolet in TFR tokamak plasmas

Line radiation in the vísible and ultraviolet (100–1300 Å) spectral regions has been studied on TFR for a 140-kA, 26-kG plasma. The emission is integrated along the line of sight, except for neutral-deuterium visible radiation, which has been spatially resolved. From the measured absolute signals, the initial (in the first ten milliseconds) amount of impurities has been determined. Subsequently, the observed signals can be interpreted in terms of an impurity-recycling flux. It has been deduced from power balance considerations that this flux is confined to an external plasma shell, a small part only diffusing towards the centre to account for the independently inferred Zeff. The power radiated by the observed lines corresponds to an important fraction of the injected Ohmic power (∼ 30-40%). Two lines ascribed to Mo XXXI and Mo XXXII (and therefore originating in the central hot plasma core) have been detected for electron temperatures greater than 1.5 keV.

Black holes and magnetic fields

The possible stationary axisymmetric electromagnetic fields in a vacuum cavity between an initially neutral black hole and a surrounding plasma shell are investigated. It is shown that such fields must be near "uniform" (in a sense defined in the paper) and that the flux of the magnetic field across one half of the surface of a neutral hole (of fixed mass) decreases as the angular momentum of the hole increases.

On the interpretation of intensity anomalies in dense aluminium plasmas

On the interpretation of intensity anomalies in dense aluminium plasmas